Photographic industry has always been working with photographic materials wherein it was desired to attain a satisfactory response to light with a well-defined minimum energy content. This light energy should trigger a chemical or physical activity leading to a change in the exposed material which should be immediately visible or, being intrinsically present, should be visualized afterwards by an additional treatment, also called processing step. Since quite a long time there exists a strong and ever lasting demand for photosensitive materials having an improved sensitivity in that said materials should respond to a more and more decreasing amount of light energy. One of the most interesting possibilities is found in photosensitive materials wherein the primary light-activated change exists on an atomic or molecular scale which in a secondary step can be multiplicated by several orders of magnitude in order to visualize the first light interaction in the material. This `two-step` mechanism of image formation mechanism is e.g. encountered in silver halide materials which form the main subject of the present invention. It is clear that sensitivity in this type of materials is determined by the efficiency the different steps (also determined by minimization of loss processes) between light interaction with silver halide and formation of the visual image can proceed with. This invention will therefore especially be focused on the way in which the first molecular light-induced change in silver halide crystals (also called latent image) is realized. Improvements in this stage should indeed be expected to give rise, after development, to the formation of a visual image having improved sensitometric characteristics.
The efficiency of the latent image formation depends on many factors and can therefore be influenced in a lot of differing ways. The best result is expected to be realized indeed if each photoelectron in the silver halide crystal reaches the deepest electron trap thereby forming a latent image. This means that recombination between holes and electrons that are created after light absorption is prevented as much as possible. Many solutions have since quite a long time been proposed but all of them have hitherto shown a limited result. One can e.g. as a prime measure try to lower the depth of an electron trap in order to increase the capture probability. Chemical sensitization with e.g. sulphur, gold, selenium and other compounds or combinations thereof has therefore often been used for this purpose.
Another way to prevent recombination of holes and electrons after their generation is the temporary interception of these species at local traps with intermediate trap depth. This can be performed by creating an internal distortion in the crystal lattice, e.g. by the local incorporation of an increased amount of iodide in the core or in a small zone or band within the grain. Although this method leads to a sensitivity gain by decreasing the electron-hole recombination, another important feature like developability, which is particularly desired in modern photographic materials for use in rapid processing systems is deteriorated by the presence of iodide.
Increasing the sensitivity of a silver halide emulsion can also be realized by increasing the efficiency of electron transfer from a spectral sensitizer to the silver halide grain which can in principle be carried out with the help of a supersensitizer.
Looking at the activity of the sensitizing dye, gain in sensitivity cannot only be realized by increasing the efficiency of dye sensitization but can also be realized by decreasing dye desensitization, wherein said desensitization is e.g. due to an increasing dye concentration at the grain surface. Combination of an electron donating compound like ascorbic acid with specific cyanine and merocyanine dyes as described in U.S. Pat. No 4,897,343 is an efficient measure in order to reach that goal. Electron-donating compounds attached to a sensitizing dye or a silver halide absorptive group have also been used to get a additional sensitizing effect.
Examples thereof have been described in U.S. Pat. Nos. 5,436,121; 5,478,719 and 4,607,006.
Another interesting way in order to make decrease recombination effects consists in the introduction of hole traps like silver clusters or defined complexes, as e.g. metal complexes, also called dopants, in the silver halide crystal. Silver clusters can be created in the crystals by reduction sensitization, therefore treating the emulsion crystals during precipitation with a reducing agent like e.g. tin compounds, polyamine derivatives, hydrazines, ascorbic acid and analogues, etc., or by creating well defined conditions in the precipitation vessel for pH- and/or pAg without requiring use of any reducing substance. So in U.S. Pat. No. 3,892,574 a method has been described, wherein during precipitation of the silver halide or before or during physical ripening small silver specks (which are so small that they do not give spontaneously developable fog) are created in reducing conditions. The same can be said for silver halide preparation methods as described in U.S. Pat. No. 3,957,490; where at the end of a reduction periode an oxidizing agent is introduced in the silver halide emulsion before chemical sensitization. Most of the methods mentioned before however give rise to fog to a lesser or larger extent so that this poses serious problems.
In all the concepts mentioned hereinbefore experimental evidence has been found that these silver clusters can easily be formed on {111}-AgBr crystal faces if compared e.g. with {100}-AgBr and {100}-AgCl crystal faces.